The invention relates generally to sound dampening techniques and more particularly to methods and apparatus for active noise cancellation.
It is often desirable to reduce the ambient noise level in a particular environment. This is particularly true when the noise is loud and unpleasant, such as the noise produced by machinery. In fact, loud noise can be more than annoying; at certain sustained levels it can cause pain and permanent injury.
Generally speaking, prolonged exposure to noise levels below about 70 decibels (dB) is perfectly sustainable to most people. When the noise level is within the range of about 70 to 90 decibels, most people will begin to experience irritation and stress. Sustained exposure to noise in the range of 90 to 120 dB can cause permanent hearing loss, and exposure to noise much in excess of 120 dB can reach the threshold of pain for most people.
The classical approach to noise reduction is to block the compression wave generated by the sound source with a sound absorbing substance. This type of noise reduction is known as passive noise reduction because it does not require an external energy source to accomplish its task. Examples of passive noise reduction include standard automobile mufflers, enclosures for noisy machinery and acoustical ceiling tile. Passive noise reduction tends to be more effective for high frequency noise than for low frequency noise.
Another approach to noise reduction is active sound reduction, which refers to any electro-acoustical method in which an undesired sound wave is canceled by a second sound wave that has the same amplitude but is 180.degree. out of phase. As shown in FIG. 1a, an undesired tone can be canceled by generating a second tone of the same amplitude and frequency, and adjusting its phase so that the peaks of one tone coincide with the valleys of the other. FIG. 1b illustrates the cancellation of wideband noise, such as that generated by an automobile, by an appropriately generated anti-noise. In practice, active noise reduction is most often used to attenuate low frequency noise and vibration and, therefore, tends to be complementary with passive noise reduction techniques. It is well known that active and passive noise reduction methods can be used together to attenuate a variety of wideband noise sources.
Active noise reduction research dates back at least as far as the 1930's. In the early days of the research success was limited by the available technology which essentially consisted of vacuum-tube-based analog circuitry. Signal processing errors caused by the inherent instability of the analog circuitry made it difficult to produce the correct anti-noise, thereby greatly limiting the effectiveness of the noise reduction. The development of semiconductor-based digital signal processing in the late 1960's provided new tools for analyzing sound waves and allowed sufficient control over the anti-noise signal to achieve moderate levels of noise reduction. Virtually all commercially available active noise reduction equipment is now based upon digital signal processing technology.
Prior art commercial applications of active noise reduction are concentrated in the areas of headsets and in the quieting of noise in heating, ventilation and air conditioning (HVAC) ducts. For example, headsets which utilize the principles of active noise reduction are manufactured by Bose Company of Framingham, Mass. Devices for quieting HVAC ducts are made by Digisonix/Nelson Industries of Stoughton, Wis.
The commercial products mentioned above have a number of characteristics in common. Firstly, all of the commercially available products cancel noise within an enclosure, chamber or waveguide. In the case of headsets, the chamber is defied as the volume of air enclosed by the earpieces of the headsets and the ears of the persons wearing the headsets. In HVAC applications the noise to be reduced propagates inside of an enclosed duct. Secondly, all commercially available active noise reduction products are single-channel devices which operate on sound waves traveling along a single path. Commercially available products are not, therefore, well adapted to provide effective noise reduction in environments which support complex multiple wavefronts, such as within large enclosures or in open spaces.
There are a great number of patent disclosures describing active noise cancellation systems. Examples of patents describing active noise cancellation methodologies for HVAC ducts include: U.S. Pat. Nos. 4,122,303; 4,171,465; 4,473,906; 4,480,333; 4,596,033; 4,665,549; 4,669,122; 4,677,676; 4,677,677; 4,783,817; 4,815,139; and 4,837,834. Some of these patents, such as U.S Pat. Nos. 4,473,906 and 4,665,549, disclose the use of multiple input microphones to detect the noise to be canceled. Others of these patents, such as U.S. Pat. Nos. 4,171,465 and 4,669,122 disclose multiple speakers used to cancel noise in a duct. U.S. Pat. No. 4,815,139 discloses both the use of multiple input microphones to sense noise and multiple speakers to cancel noise in a duct. Other examples of active noise cancellation patents include U.S. Pat. No. 4,637,048 which teaches the cancellation of noise from an automobile tail pipe, and U.S. Pat. Nos. 4,562,589, 4,689,821 and 4,715,559 which teach the cancellation of noise in the fuselage or cockpit of aircraft.
These patents share the same limiting characteristics as the above-mentioned commercial products: they all operate on noise within enclosed spaces such as ducts or airplane fuselages, and they all disclose single-channel cancellation devices. Even the patents which disclose multiple input microphones and/or multiple output speakers are single-channel devices in that the signals obtained from the multiple input microphones and the signals delivered to the multiple speakers are processed within a single-channel processing device. In consequence, prior art active noise cancellation devices are not well adapted to the creation of large quiet zones in open spaces or in large enclosed spaces.